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handlers.py
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handlers.py
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"""
Apparatus for consuming 'vanilla' AMQP messages (i.e. not Celery tasks) making use
of the battle-tested `bin/celery worker` utility rather than writing our own.
NOTE:
We don't access any Celery config in this file. That is because the config is
loaded by the bin/celery worker itself, according to usual mechanism.
"""
import logging
import traceback
import six
from typing import Any, Callable, Dict, Iterable, List, Optional, Union # noqa
import amqp # noqa
import celery.bootsteps as bootsteps
import kombu
import kombu.message
import kombu.common as common
from event_consumer.conf import settings
from event_consumer.errors import InvalidQueueRegistration, NoExchange, PermanentFailure
from event_consumer.types import HandlerRegistration, QueueKey
if settings.USE_DJANGO:
from django.core.signals import request_finished
_logger = logging.getLogger(__name__)
# Maps routing-keys to handlers
REGISTRY = {} # type: Dict[QueueKey, HandlerRegistration]
DEFAULT_EXCHANGE = 'default'
def _validate_registration(register_key): # type: (QueueKey) -> None
"""
Raises:
InvalidQueueRegistration
"""
global REGISTRY
if register_key in REGISTRY:
raise InvalidQueueRegistration(
'Attempted duplicate registrations for messages with the queue name '
'"{queue}" and exchange "{exchange}"'.format(
queue=register_key.queue,
exchange=register_key.exchange,
)
)
def message_handler(routing_keys, # type: Union[str, Iterable]
queue=None, # type: Optional[str]
exchange=DEFAULT_EXCHANGE, # type: str
queue_arguments=None, # Optional[Dict[str, object]]
):
# type: (...) -> Callable[[Callable], Any]
"""
Register a function as a handler for messages on a rabbitmq exchange with
the given routing-key. Default behaviour is to use `routing_key` as the
queue name and attach it to the 'default' exchange. If this key is not
present in `settings.EXCHANGES` with your own config then you will get the
underlying AMQP default exchange - this has some restrictions (you cannot
bind custom queue names, only auto-bound same-as-routing-key queues are
possible).Kwargs
Otherwise Queues and Exchanges are automatically created on the broker
by Kombu and you don't have to worry about it.
Kwargs:
routing_keys: The routing key/s of messages to be handled by the
decorated task.
queue: The name of the main queue from which messages
will be consumed. Defaults to '{QUEUE_NAME_PREFIX}{routing_key}`
if not supplied. Special case is '' this will give you Kombu
default queue name without prepending `QUEUE_NAME_PREFIX`.
exchange: The AMQP exchange config to use. This is a *key name*
in the `settings.EXCHANGES` dict.
queue_arguments: Arbitrary arguments to be passed to the *primary* queue
at creation time.
Returns:
Callable: function decorator
Raises:
InvalidQueueRegistration
Usage:
@message_handler('my.routing.key', 'my.queue', 'my.exchange')
def process_message(body):
print(body) # Whatever
Note that this is an import side-effect (as is Celery's @task decorator).
In order for the event handler to be registered, its containing module must
be imported before starting the AMQPRetryConsumerStep.
"""
if (queue or (queue is None and settings.QUEUE_NAME_PREFIX)) \
and exchange not in settings.EXCHANGES:
raise InvalidQueueRegistration(
"You must use a named exchange from settings.EXCHANGES "
"if you want to bind a custom queue name."
)
if isinstance(routing_keys, six.string_types):
routing_keys = [routing_keys]
else:
if queue is not None:
raise InvalidQueueRegistration(
"We need a queue-per-routing-key so you can't specify a "
"custom queue name when attaching mutiple routes. Use "
"separate handlers for each routing key in this case."
)
queue_arguments = queue_arguments or {}
def decorator(f): # type: (Callable) -> Callable
global REGISTRY
for routing_key in routing_keys:
queue_name = (settings.QUEUE_NAME_PREFIX + routing_key) if queue is None else queue
# kombu.Consumer has no concept of routing-key (only queue name) so
# so handler registrations must be unique on queue+exchange (otherwise
# messages from the queue would be randomly sent to the duplicate handlers)
register_key = QueueKey(queue=queue_name, exchange=exchange)
_validate_registration(register_key)
handler_registration = HandlerRegistration(
routing_key=routing_key,
queue_arguments=queue_arguments,
handler=f,
)
REGISTRY[register_key] = handler_registration
_logger.debug(
'registered: %s to handler: %s.%s',
register_key,
f.__module__,
f.__name__
)
return f
return decorator
class AMQPRetryConsumerStep(bootsteps.StartStopStep):
"""
An integration hook with Celery which is adapted from the built in class
`bootsteps.ConsumerStep`. Instead of registering a `kombu.Consumer` on
startup, we create instances of `AMQPRetryHandler` passing in a channel
which is used to create all the queues/exchanges/etc. needed to
implement our try-retry-archive scheme.
See http://docs.celeryproject.org/en/latest/userguide/extending.html
"""
requires = ('celery.worker.consumer:Connection', )
def __init__(self, *args, **kwargs):
self.handlers = [] # type: List[AMQPRetryHandler]
self._tasks = kwargs.pop('tasks', REGISTRY) # type: Dict[QueueKey, HandlerRegistration]
super(AMQPRetryConsumerStep, self).__init__(*args, **kwargs)
def start(self, c):
channel = c.connection.channel()
self.handlers = self.get_handlers(channel)
for handler in self.handlers:
handler.declare_queues()
handler.consumer.consume()
_logger.debug('AMQPRetryConsumerStep: Started handler: %s', handler)
def stop(self, c):
self._close(c, True)
def shutdown(self, c):
self._close(c, False)
def _close(self, c, cancel_consumers=True):
channels = set()
for handler in self.handlers:
if cancel_consumers:
common.ignore_errors(c.connection, handler.consumer.cancel)
if handler.consumer.channel:
channels.add(handler.consumer.channel)
for channel in channels:
common.ignore_errors(c.connection, channel.close)
def get_handlers(self, channel):
return [
AMQPRetryHandler(
channel=channel,
routing_key=handler_registration.routing_key,
queue=queue_key.queue,
exchange=queue_key.exchange,
queue_arguments=handler_registration.queue_arguments,
func=handler_registration.handler,
backoff_func=settings.BACKOFF_FUNC,
)
for queue_key, handler_registration in self._tasks.items()
]
class AMQPRetryHandler(object):
"""
Implements Depop's try-retry-archive message queue pattern.
Briefly - messages are processed and may be retried by placing them on a separate retry
queue on a dead-letter-exchange. Messages on the DLX are automatically re-queued by Rabbit
once they expire. The expiry is set on a message-by-message basis to allow exponential
backoff on retries.
"""
def __init__(self,
channel, # type: amqp.channel.Channel
routing_key, # type: str
queue, # type: str
exchange, # type: str
queue_arguments, # type: Dict[str, str]
func, # type: Callable[[Any], Any]
backoff_func=None # type: Optional[Callable[[int], float]]
):
# type: (...) -> None
self.channel = channel
self.routing_key = routing_key
self.queue = queue # queue name
self.exchange = exchange # `settings.EXCHANGES` config key
self.func = func
self.backoff_func = backoff_func or self.backoff
self.exchanges = {
DEFAULT_EXCHANGE: kombu.Exchange(channel=self.channel)
}
for name, exchange_settings in settings.EXCHANGES.items():
self.exchanges[name] = kombu.Exchange(
channel=self.channel,
**exchange_settings
)
try:
self.worker_queue = kombu.Queue(
name=self.queue,
exchange=self.exchanges[exchange],
routing_key=self.routing_key,
channel=self.channel,
queue_arguments=queue_arguments,
)
self.retry_queue = kombu.Queue(
name='{queue}.retry'.format(queue=queue),
exchange=self.exchanges[DEFAULT_EXCHANGE],
routing_key='{queue}.retry'.format(queue=queue),
# N.B. default exchange automatically routes messages to a queue
# with the same name as the routing key provided.
queue_arguments={
"x-dead-letter-exchange": "",
"x-dead-letter-routing-key": self.queue,
},
channel=self.channel,
)
self.archive_queue = kombu.Queue(
name='{queue}.archived'.format(queue=queue),
exchange=self.exchanges[DEFAULT_EXCHANGE],
routing_key='{queue}.archived'.format(queue=queue),
queue_arguments=settings.ARCHIVE_QUEUE_ARGS,
channel=self.channel,
)
except KeyError as key_exc:
raise NoExchange(
"The exchange {exchange} was not found in settings.EXCHANGES.\n"
"settings.EXCHANGES = {exchanges}".format(
exchange=key_exc,
exchanges=settings.EXCHANGES
)
)
self.retry_producer = kombu.Producer(
channel,
exchange=self.retry_queue.exchange,
routing_key=self.retry_queue.routing_key,
serializer=settings.SERIALIZER,
)
self.archive_producer = kombu.Producer(
channel,
exchange=self.archive_queue.exchange,
routing_key=self.archive_queue.routing_key,
serializer=settings.SERIALIZER,
)
self.consumer = kombu.Consumer(
channel,
queues=[self.worker_queue],
callbacks=[self],
accept=settings.ACCEPT,
)
self.consumer.qos(prefetch_count=settings.PREFETCH_COUNT)
def __repr__(self):
return (
"AMQPRetryHandler("
"routing_key={routing_key}, "
"queue={queue}, "
"exchange={exchange}, "
"func={func.__module__}.{func.__name__}"
")".format(
routing_key=self.routing_key,
queue=self.queue,
exchange=self.exchange,
func=self.func,
)
)
def __call__(self, body, message):
"""
Handle a vanilla AMQP message, called by the Celery framework.
Raising an exception in this method will crash the Celery worker. Ensure
that all Exceptions are caught and messages acknowledged or rejected
as they are processed.
Args:
body (Any): the message content, which has been deserialized by Kombu
message (kombu.message.Message)
Returns:
None
"""
retry_count = self.retry_count(message)
try:
_logger.debug(
'Received: (key={routing_key}, retry_count={retry_count})'.format(
routing_key=self.routing_key,
retry_count=retry_count,
)
)
self.func(body)
except Exception as e:
if isinstance(e, PermanentFailure):
self.archive(
body,
message,
"Task '{routing_key}' raised '{cls}, {error}'\n"
"{traceback}".format(
routing_key=self.routing_key,
cls=e.__class__.__name__,
error=e,
traceback=traceback.format_exc(),
)
)
elif retry_count >= settings.MAX_RETRIES:
self.archive(
body,
message,
"Task '{routing_key}' ran out of retries ({retries}) on exception "
"'{cls}, {error}'\n"
"{traceback}".format(
routing_key=self.routing_key,
retries=retry_count,
cls=e.__class__.__name__,
error=e,
traceback=traceback.format_exc(),
)
)
else:
self.retry(
body,
message,
"Task '{routing_key}' raised the exception '{cls}, {error}', but there are "
"{retries} retries left\n"
"{traceback}".format(
routing_key=self.routing_key,
retries=settings.MAX_RETRIES - retry_count,
cls=e.__class__.__name__,
error=e,
traceback=traceback.format_exc(),
)
)
else:
message.ack()
_logger.debug(
"Task '{routing_key}' processed and ack() sent".format(routing_key=self.routing_key)
)
finally:
if settings.USE_DJANGO:
# avoid various problems with db connections, due to long-lived
# worker not automatically participating in Django request lifecycle
request_finished.send(sender="AMQPRetryHandler")
if not message.acknowledged:
message.requeue()
_logger.critical(
"Messages for task '{routing_key}' are not sending an ack() or a reject(). "
"This needs attention. Assuming some kind of error and requeueing the "
"message.".format(routing_key=self.routing_key)
)
def retry(self, body, message, reason=''):
"""
Put the message onto the retry queue
"""
_logger.warning(reason)
try:
retry_count = self.retry_count(message)
headers = message.headers.copy()
headers.update({
settings.RETRY_HEADER: retry_count + 1
})
self.retry_producer.publish(
body,
headers=headers,
retry=True,
declares=[self.retry_queue],
expiration=self.backoff_func(retry_count)
)
except Exception as e:
message.requeue()
_logger.error(
"Retry failure: retry-reason='{reason}' "
"exception='{cls}, {error}'\n"
"{traceback}".format(
reason=reason,
cls=e.__class__.__name__,
error=e,
traceback=traceback.format_exc(),
)
)
else:
message.ack()
_logger.debug("Retry: {reason}".format(reason=reason))
def archive(self, body, message, reason=''):
"""
Put the message onto the archive queue
"""
_logger.warning(reason)
try:
self.archive_producer.publish(
body,
headers=message.headers,
retry=True,
declares=[self.archive_queue],
)
except Exception as e:
message.requeue()
_logger.error(
"Archive failure: retry-reason='{reason}' "
"exception='{cls}, {error}'\n"
"{traceback}".format(
reason=reason,
cls=e.__class__.__name__,
error=e,
traceback=traceback.format_exc(),
)
)
else:
message.ack()
_logger.debug("Archive: {reason}".format(reason=reason))
def declare_queues(self):
queues = [self.worker_queue, self.retry_queue, self.archive_queue]
for queue in queues:
queue.declare()
@classmethod
def retry_count(cls, message):
return message.headers.get(settings.RETRY_HEADER, 0)
@staticmethod
def backoff(retry_count):
# type: (int) -> float
"""
Given the number of attempted retries at delivering a message, return
an increasing TTL for the message for the next retry (in seconds).
"""
# First retry after 200 ms, then 1s, then 1m, then every 30m
retry_delay = [0.2, 1, 60, 1800]
try:
return retry_delay[retry_count]
except IndexError:
return retry_delay[-1]